Presently, low pressure reservoirs, incapable of producing fluid from the reservoir to the surface naturally, account for a majority of the hydrocarbon producing wells in the United States. There are various means of pumping fluid from these wells, such as the use of sucker rod pumps, hydraulic pumps, jet pumps, and semi-submersible electric pumps. Most of these depleted wells produce fluid at pressure and flow rates too low for the majority of existing pumps to operate efficiently and/or economically.
The most common method used for producing these low pressure, low flow rate wells is the use of sucker rod pumping systems. Sucker rod pumping systems include a downhole plunger and cylinder type pump, connected to a surface unit (e.g., a pump jack) by connecting rods (e.g., sucker rods). Existing sucker rod systems include multiple limitations and difficulties inherent in their use. While the stroke length of the pump and the stroke frequency may be controlled through the selection of the pump jack size, pumping jacks are too costly, and each pump size is limited to a specific range of flow rates and depth of the reservoir. Once a pump unit is placed, it is cost prohibitive to change the pump jack, thus modification of stroke length and/or frequency is often impossible. Another large problem with conventional sucker rod systems relates to the sucker rods, themselves. Sucker rods include segments of metal or fiberglass rod that are connected together to form a continuous string of rods, normally several thousand feet in length when used in hydrocarbon wells. These rod strings are typically connected using pin and box connections (e.g., threaded connections). The process of connecting the rod string when running sucker rod segments into a wellbore, or disconnecting the string when removing rod segments from the wellbore, is time consuming and costly. Additionally, the length and weight of these rods and the repeated reciprocation of the rods caused by the pump jack often results in failure, commonly by a parting of the sucker rod string. Another difficulty associated with the use of sucker rod strings is the position of the rod string within a tubing string (e.g., production tubing). When the system is operating, it is common for the rod string to contact the inner wall of the tubular string at various points, which results in wear of both the rod string and the tubular string, and can eventually cause failure of the well tubing string, as well as the rod string. Depending of the severity of the wellbore conditions, rod pumping systems fail on the average of once a month, quarterly, or semiannually, requiring significant repair and maintenance costs. The frequency and expense of necessary repairs and maintenance is often a significant factor that causes production of a well to become uneconomical. Failure rates in rod pumping systems are significantly more common in deviated and/or non-vertical wellbores.
There have been attempts to develop a pumping system which utilizes a plunger/cylinder-type downhole pump while eliminating the use of sucker rods, thereby eliminating the problems inherent in the use of sucker rods. Existing rodless pump systems typically include a surface unit, which is connected to a subsurface pump by a fluid conduit, such as the tubing string. The surface unit activates the subsurface pump by applying pressure to the fluid in the tubing string to compress a spring or similar member in the subsurface pump and displace a slidable piston, which thereby draws fluid from the wellbore into a pump chamber. When the surface unit releases the fluid pressure, a spring mechanism in the subsurface pump will displace the piston and lift the fluid from the pump chamber into the tubing string and toward the surface. Although, such systems eliminate use of a sucker rod string, they require a compression spring for lifting the produced fluid into the tubing string. Use of such a spring severely limits the stroke length and thus, the flow rate of the pump. Further, springs used in this manner tend to fail due to wear and/or the accumulation of debris carried into the pump.
Other existing rodless pumps replace the physical spring with a gas chamber. When pressure is applied to the tubing string, a piston will compress the gas within the chamber, and when the pressure is relieved, the gas will expand to lift fluid into the tubing string. These systems allow for a longer stroke length and thus much higher efficiency, but introduce additional problems. A major problem inherent in the use of rodless pumps is that, unlike sucker rod pumps, a rodless pump does not have a precisely defined stroke length. In a rodless pump, the stroke length is affected by the length of time the surface unit applies pressure to the fluid in the tubing string during each cycle, by the compressibility of the fluid in the tubing string, and by the amount of ballooning of the tubing that occurs. The stroke length is also influenced by the pressure in the gas chamber, since the pressure in the gas chamber must be sufficient to support the hydrostatic pressure of the entire column of fluid extending to the surface. At the end of each downstroke, enough force is applied to the plunger to cause the plunger to strike the bottom of the barrel with a significant impact, causing excessive wear and potential damage. Also, because the surface unit is unable to stop applying pressure to the tubing string at the precise moment necessary to prevent this contact, the plunger will also impact the limit stop at the end of each upstroke. Thus, unlike sucker rod pumps, rodless pumps are difficult to design in a manner that enables the maximum stroke to be utilized without the plunger contacting the barrel at the end of the upstroke and downstroke, severely limiting the usable life of such pumps.
Other rodless pumps attempt to overcome these severe plunger impacts through use of dampening mechanisms, such as elastomer barriers, springs, and/or other types of dampeners, at both the top and bottom of the plunger's stroke. However, such rodless pump systems still utilize a downhole gas source within the pump to force the plunger assembly downward after the surface pressure source releases the pressure being exerted on the downhole pump. The gas pressure source requires a substantially self-contained pressure chamber, which can be part of the pump, can be positioned downhole, and can be used to contain a substantially compressible fluid. The pressure chamber can be precharged with a gas, such as nitrogen. Although this arrangement is an improvement over preceding pumps, particularly those subject to plunger impact, it still possesses inherent limitations. For example, this arrangement of pump requires a very high precharge pressure in the gas chamber, the pump will suffer from a short piston life due to fluid leakage and contamination, and the pump will require bleeding the substantial gas chamber pressure whenever retrieving the pump to the surface.
Embodiments usable within the scope of the present disclosure improve upon these and other existing designs by eliminating the use of rods, pump jacks, springs, and downhole gas sources or gas pressure chambers within the pump to meet the need for a rodless pump that is operable downhole without plunger impact problems and having a substantial usable life.
Another limitation associated with existing pumps is the requirement of a housing structure, which surrounds sections of the pump, as a means of engagement. To install such a pump, the tubing string, such as production tubing, must be extracted from the well, such that the pump can be connected at the end of the tubing string (e.g., via threading the housing to the tubing). The pump is then lowered into the well by lowering the tubing string. This undertaking requires a significant quantity of manual labor and well downtime, resulting in significant costs and losses of revenues. Furthermore, most repairs to these types of pumps also require the extraction of the entire tubular string to access the pump, which requires a major rig to handle the weight.
Embodiments usable within the scope of the present disclosure improve upon these and other existing designs by eliminating the use of wide housing, thereby meeting the need for a subsurface pump that can be inserted and extracted from and/or through the tubing string without requiring extraction of the tubing string itself.
However, pumps that do not contain a housing structure, and are inserted directly into the existing tubing string, can be faced with certain problems. Because such pumps have small barrel and plunger diameters, they are normally capable of moving only small volumes of produced hydrocarbons with each stroke. One system that can overcome this limitation is a system that includes a pump with an increased stroke length. Pumps having longer stroke lengths, however, can be encumbered with problems, such as piston shaft buckling, ineffective sealing between the pistons and the pump barrel, and significant barrel strains due to deep well pressures. Embodiments usable within the scope of the present disclosure improve upon existing systems and methods of use to meet the needs for a subsurface pump having an increased stroke length, which is operable downhole without piston shaft buckling and problems associated with sealing and barrel deformation.